The present invention relates to a method and circuit arrangement for synchronizing the carrier frequency of a reference station with the local oscillator frequency of each of a plurality of remote stations.
While not limited to such a communication system, the instant invention is directed to the frequency control of remote stations with respect to a reference ground station in a single channel per carrier (SCPC) satellite communication system. Frequency stability is important for SCPC systems because it tends to prevent adjacent channel interference (ACI) which may occur when inadequate separation of the channels is attained; further, it tends to prevent the frequency offset of the carrier from exceeding the demodulator pull-in range. The purpose of a frequency control technique is to correct the frequency errors introduced by the frequency offset of oscillators in the transmission path, including those contributed by the satellite transponders.
Many different frequency control techniques have been used to compensate for the frequency offset which occurs during satellite transmission, and a number of these techniques employ either one or two pilot frequencies. One technique that employs two pilot frequencies is disclosed in U.S. Pat. No. 3,450,842, inventor D. E. W. Lipke, entitled, "Doppler Frequency Spread Correction Device For Multiplex Transmissions", granted June 17, 1969. In this control circuit the frequency difference between the untransmitted pilots is compared to the frequency difference between the received pilots. The reason a comparison is made between the pilots transmitted from a remote station and the local untransmitted difference in pilot frequencies is to change the local oscillator transmitter frequency so as to correct for doppler shift caused by movement of the transmitting and receiving stations relative one to the other.
In U.S. Pat. No. 4,188,579, inventor Akiyuki Yoshisato, entitled, "SSB Transceiver", granted Feb. 12, 1980, the inventor also makes use of two pilot frequencies to obtain frequency correction. In this case the pilot frequencies sent from the one terminal are recovered at the receiver of the other, "remote", terminal. The recovered pilots are used to actuate a control circuit which, in turn, controls a saw-tooth waveform generator which has a continually varying frequency, and is a part of an automatic frequency tuner. The control circuit acts to stop the voltage generator when the demodulated pilot signals have a frequency which is equal to the predetermined frequency, i.e., the absolute values of the frequencies have not be shifted. This is accomplished by the use of two pilot frequency filters which are very narrow band and thus substantially only pass the design frequency. When the pilots are off frequency the filters block transmission. A logic circuit recognizes this condition and the voltage sweep is allowed to change the frequency of the local oscillator. This will continue until the local oscillator frequency is at a frequency that positions the pilot frequencies in the passband of their respective filters. Once the pilot frequencies are passed to the logic circuit, the activity of the voltage sweep is terminated and the local oscillator is held at that frequency until the pilot frequencies are again blocked by the pilot filters.